The present invention relates in general to preimpregnated materials.
As is known, pre-impregnated materials (also commonly known as prepregs) are composite materials used in various industrial sectors, and in particular in the aeronautical industry. A prepreg is generally a semi-finished product comprising reinforcing fibers and a resin matrix in which these fibers are embedded. The fibers may be arranged in different configurations, for example in a unidirectional layer, in two layers having orientations different from each other, or as fabric. Prepregs are generally prepared in the form of strips and wound in rolls.
The preimpregnated materials mainly used in the aeronautical industry have a thermosetting resin matrix which, before polymerization, has tackiness characteristics. They can therefore be used to form laminates by placing various layers on top of each other, with a suitable orientation sequence of various layers. The laminates are then subjected to a temperature and pressure cycle (in a vacuum bag and in an autoclave) which causes the material to polymerize, increasing its molecular weight and promoting the formation of bonds between the macromolecules (crosslinking), transforming it into a material with structural characteristics suitable for working use.
The laminates may also be made using automated methods which involve significant advantages in terms of cost, productivity and repeatability. For flat or moderately curved laminations, an apparatus known as an automatic tape layer is used. Recently, a technique has become established where curved surfaces, including closed (cylindrical) surfaces, may be laminated using preimpregnated strips with a relatively small width (called slits); this technique is known as Automatic Fiber Placement and is already used for composite parts with a thermosetting matrix.
Prepregs with a matrix of thermoplastic material are also used in the aeronautical industry. In the case of prepregs based on thermoplastic materials, the resin has a high molecular weight and therefore, on the one hand, it does not need to be subjected to a polymerization cycle while, on the other hand, it does not have tackiness characteristics. A prepreg with thermoplastic matrix may, as a first approximation, be regarded as an article in its finished state formed by a single lamina. In order to be able to form a laminate, it must be heated so as to cause melting of the thermoplastic preimpregnated plies which form it, compacted under pressure and then allowed to cool. The temperature to be reached for melting is the glass transition temperature Tg for amorphous thermoplastics and the melting point Tf for semi-crystalline thermoplastics.
At present the typical processing technique for composite aeronautical parts with a thermosetting matrix comprises manual or automated lamination of the plies of material followed by application of a vacuum bag and a curing cycle in an autoclave. In the case of thermosetting materials, the curing cycle must ensure both consolidation of the part and its chemical transformation (polymerization and crosslinking). This cycle therefore requires raising of the temperature at a controlled speed (about 2-3° C./min), a period of exposure to high temperature (typically about 2 hours at 180° C.) and cooling. In the case of thermoplastics the aforementioned process is also possible, but in this case heating is required only in order to render the part fluid (above the glass transition temperature Tg for amorphous thermoplastics and above the melting point Tf for semi-crystalline thermoplastics); the pressure is used, as in the case of the thermosetting materials, to consolidate the plies. In the case of semi-crystalline materials, it is necessary to have a very high consolidation temperature (e.g. above 350° C. for composites with a polyether ether ketone (PEEK) matrix) which requires a high temperature in an autoclave, specific materials for the process (heat-resistant vacuum bag film, high-temperature sealing tape, etc.) and therefore involves high costs for energy, equipment and service materials used, etc. The cooling rate must also be controlled in order to ensure that it is within the correct working range, so as to obtain the correct degree of crystallinity.